1. Field of the Invention
The present invention relates to a technique for determining a proper print speed in an inkjet printer.
2. Description of the Background Art
An inkjet printer includes pressure generating elements such as piezoelectric elements and electrothermal converters for the purpose of ejecting ink from a plurality of nozzles provided in a head. For this reason, the temperature of the head is increased by the drive load of the pressure generating elements at the time of printing. There is apprehension that the increase in the temperature of the head gives rise to problems including the decrease in image quality resulting from changes in physical properties of ink, damages to driving circuits of the pressure generating elements, and the like.
A known solution to such problems is the technique of suspending printing when the temperature of the head reaches a predetermined value or higher. This technique is disclosed, for example, in Japanese Patent Application Laid-Open No. 11-179893 (1999), paragraphs 0003 to 0006. The suspension of printing, however, causes problems such as the decrease in productivity and the difficulty in providing uniform image quality.
The greater the drive load of the pressure generating elements is, the more significantly the increase in the temperature of the head occurs. That is, the greater the amount of ink ejected toward printing paper is, the more prone to increase the temperature of the head is. Also, the continuous execution of printing causes the temperature of the head to increase gradually. Thus, even after several pages are successfully printed at a certain print speed without any problem, there are cases in which further continuous printing causes the temperature of the head to exceed a permissible value. It is, however, not preferable from the viewpoint of productivity to print at all times at an extremely low speed such that the continuous printing of 100% solidly shaded images for a long time cause no problem.
It is also contemplated to detect the temperature inside the head at the time of printing to change the print speed during printing, based on the detected temperature. However, changing the print speed during printing results in complicated control for maintaining constant printing precision and constant image quality.
To solve these problems, Japanese Patent Application Laid-Open No. 11-179893 (1999) discloses the technique of calculating a print density from information in a print buffer to determine the print speed in accordance with the calculated print density for the purpose of printing at a constant speed without the suspension of printing.
Unfortunately, an inkjet printer including a plurality of heads supplies print data to the heads in a distributed manner. This makes it impossible to expect the elevated temperature condition of each head by only referencing the print density of the entire print data. Also, when a plurality of blocks each serving as a control unit are provided in one head, there are differences in elevated temperature condition between the blocks. In such a case, it is also impossible to expect the elevated temperature condition of each block by only referencing the print density of the entire print data.
Japanese Patent Application Laid-Open No. 2005-199445 discloses the technique of determining the maximum value of the print speed by using the maximum value of the amount of ink ejection per unit area in an inkjet printer including an elongated head because a partial temperature increase is expected to occur in a region where a large amount of ink is ejected per unit area although the total number of times of ink ejection is not necessarily high (see paragraphs 0082 to 0084).
In a head having a plurality of blocks arranged widthwise, each of the blocks is not only increased in temperature spontaneously by the ejection of ink but also influenced by the increase in temperature of its surrounding blocks. Only referencing the maximum value of the amount of ink ejection per unit area as in Japanese Patent Application Laid-Open No. 2005-199445 cannot achieve the determination of a proper print speed in consideration for the thermal influence of the surrounding blocks.